The so-called “photocatalytic” reaction, also called photocatalysis, consists in destroying the various organic and/or inorganic pollutants present in gaseous effluents, and in particular air, or in liquid effluents by photochemical reaction, which reaction is caused by the irradiation of a photocatalyst with ultraviolet radiation.
Photocatalysis is essentially initiated by activating a semiconductor solid (photocatalytic agent such as for example TiO2) with UV radiation at a wavelength of less than 380 nanometers, causing electron changes in the semiconductor and leading, in the presence of gaseous or liquid effluents, such as air or water, to the creation of oxygenated and hydroxyl radicals at the surface of the semiconductor. These radicals attack the organic compounds adsorbed on the semiconductor and, through a succession of chemical reactions, degrade the compounds up to the final stage of oxidation.
As already stated, as photocatalytic agent capable of triggering the photocatalytic reaction, it is possible to use in particular, but without limitation, titanium dioxide TiO2 anatase, which, activated by UV light, becomes electronically modified in such a manner as to lead to the formation of hydroxyl OH and oxygen O radicals capable of attacking the organic carbon chains adsorbed on TiO2, degrading them until the organic carbon is completely converted to carbon dioxide.
It is possible, however, to envisage other photocatalytic agents such as those in particular forming part of the group consisting of metal oxides, alkaline-earth oxides, actihide oxides and rare-earth oxides. In practice, the photocatalytic agents are attached by means of binding agents to supports, such as in particular natural or synthetic nonwovens, glass fibers or metal or plastic grids.
Particularly advantageous photocatalytic compositions resulting from the mixing of a photocatalytic agent and the binding agent are described in the document WO 99/51345 by the Applicant.
These compositions may be used for the treatment of air. Indeed, air can contain a large number of pollutants, among which NOx, NH3, H2S, CO, O3, chlorinated or nonchlorinated C2-C4 alkenes, chloromethane, isooctane, benzene, toluene, xylene, isopropylbenzene, saturated aliphatic C1-C4 alcohols, methyl mercaptan, chlorophenol, nitrophenol, methyl tert-butyl ether, dimethoxymethane, C1-C4 aldehydes, acetone, formic acid, acetic acid, 2-methylpropanoic acid, dichloroacetyl chloride, dimethylformamide, trimethylamine, acetonitrile, pyridine, methanethiol, dimethyl disulfide.
The photocatalytic compositions may also be used for the treatment of liquid effluents, in particular of water polluted with organic compounds, bacteria, viruses, microbes, and the like.
In the abovementioned document WO 99/51345, the Applicant described a filtering medium for the treatment of air or water, consisting of a support coated with a composition resulting from a mixture of binding agent, photocatalytic agent and activated carbon. As explained in this document, the aim of this type of filtering medium is:                first of all, because of the high specific surface area of activated carbon, to adsorb the polluting agents present in the pollution peaks;        and then, by photocatalytic reaction under ultraviolet radiation, to degrade the pollutants adsorbed on the activated carbon, by desorption onto the photocatalytic agent, thus allowing the regeneration of said carbon.        
Although, as indicated, the intimate mixing of activated carbon and a photocatalytic agent makes it possible to increase the life of the medium, saturation of the sites of the activated carbon with the pollutant is inevitably observed because, in particular, the activated carbon present at the surface of the layer prevents the UV radiation from reaching the depth of the photocatalytic agent. In addition, nothing is indicated regarding the mass of the layer of activated carbon and the proportion of activated carbon it incorporates, which is necessary for proper depollution without saturation. In other words, the photocatalytic reaction is not sufficiently effective to allow proper regeneration of the activated carbon, such that it is necessary to replace the medium saturated with its pollutants with fresh medium relatively frequently. Quite obviously, disposal of the filtering medium charged with its pollutants is not without consequence on the environment.
The document JP-11179118 describes a filtering medium in laminated form, resulting from the combination of a layer of activated carbon with a layer of photocatalytic agent. Here again, nothing is indicated relating to the mass of activated carbon-based layer and the proportion of activated carbon necessary to allow effective depollution while avoiding saturating said activated carbon.